BoltClock's answer is correct. The only thing I want to append is that if you want to only select the pseudo element, put in a span.
For example:
<li><span data-icon='u'></span> List Element </li>
instead of:
<li> data-icon='u' List Element</li>
This way you can simply say
ul [data-icon]:hover::before {color: #f7f7f7;}
which will only highlight the pseudo element, not the entire li element